An ancient vegetable called celery root, celery knob, or celeriac is a variety of celery developed for its enlarged root rather than for stalks and leaves. It is highly prized in Europe either raw or cooked. The vegetable deserves to be better known in America, as it is nutritious and has an appealing flavor. In this recipe for a classic French pre-luncheon hors d'oeuvre, the raw celery root is marinated in a flavorful remoulade sauce.